Long-Term Evolution (LTE) of UTRAN (also denoted E-UTRAN) uses OFDM in the downlink and DFT-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. In the time domain (cf. FIG. 2), LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length T.sub.subframe=1 ms for frequency division duplex (FDD).
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e. in each subframe the base station transmits control information about to which terminals data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. A downlink system with 3 OFDM symbols as control is illustrated in FIG. 3.
LTE uses hybrid-ARQ, where, after receiving downlink data in a subframe, the terminal attempts to decode it and reports to the base station whether the decoding was successful (ACK) or not (NAK). In case of an unsuccessful decoding attempt, the base station can retransmit the erroneous data.
Uplink control signaling from the terminal to the base station consists of hybrid-ARQ acknowledgements for received downlink data;
terminal reports related to the downlink channel conditions, used as assistance for the downlink scheduling;
scheduling requests, indicating that a mobile terminal needs uplink resources for uplink data transmissions.
There are two different cases for transmitting uplink control signaling and which to use depends on whether the terminal is simultaneously transmitting data in the uplink. Both of the methods are supported by all terminals
(a) No simultaneous transmission of data and control:
In case the terminal does not transmit data at the same time as control, control signaling is transmitted on the Physical Uplink Control Channel (PUCCH). The resource used for control channel transmissions is either indicated by the downlink transmission from with the control information resides or is semi-statically configured. With the exception of an aperiodic CSI (CQI/PMI/RI) scheduled without data transmission (CQI-only). Then the PUSCH will be used with the resource indicated in the grant message.
(b) Simultaneous transmission of data and control:
In case the terminal simultaneously need to transmit uplink control information and data, control and data are multiplexed prior to transmission and transmitted on the Physical Uplink Shared Channel (PUSCH).
If the mobile terminal has been assigned an uplink resource for data transmission and at the same time instance has control information to transmit, it will transmit the control information together with the data. The control information can consists of ACK/NAK feedback for the downlink transmission, channel quality indicator (CQI), precoding matrix indicator PMI and rank indicator (RI). FIG. 4 illustrates the position of data and the different control types in one subframe from one mobile terminal. As illustrated in FIG. 4, CQI and PMI is jointly coded and uses the same set of resource elements. Scheduling requests are not transmitted together with uplink data, instead is a buffer status report transmitted jointly coded with the data if it has been triggered. CQI/PMI can be requested by a bit in the uplink grant. The CQI/PMI format used for requested reports is frequency selective while periodic reports configured on PUCCH is smaller but often non-selective.
The different control types are multiplexed with data differently. The amount of resources used for CQI/PMI and RI is taken into account when data is placed in the subframe so that data is only placed at the positions not allocated by CQI/PMI and RI. The ACK/NACK then overwrites the data and possible also CQI/PMI.
The amount of data that is transmitted within assigned resource blocks in uplink is indicated by the number of resource blocks assigned and a modulation coding scheme, as illustrated in FIG. 5. One of the principles behind this design is that the amount of resources blocks assigned to the uplink transmission should be independent from the set of possible code rates, i.e. the same code rates for data is available independent from the number of resource blocks assigned (slight differences exists).
The amount of control information that is transmitted together with data is determined by the code rate of data, without taking the amount of control information into account. Since the data on PUSCH is protected by the HARQ protocol it is usually operated at a high error rate than required for the ACK/NAK, CQI/PMI and RI. Therefore it is possible to configure an offset so that control is given a lower code rate than data by a certain number of dB.
It is possible to send reports regarding downlink channel condition on PUSCH also without data, indicated by a special point in the MCS table. If other control information is multiplexed with such a transmission the code rate of that control information is dependent on the code rate of CQI/PMI instead of data.
The CQI/PMI fields is the by far the large possible control field as it can consist of as much as 5000 coded bits, as compared to the RI and ACK/NACK which is up to about 400 coded bits.
In 3GPP contribution R1-082923, there is proposed to adopt the definition of code rate in the control information MCS calculation formulation. The proposed definition of code rate is based on the payload size of data including any CRC, the payload size of CQI/PMI including CRC if any, the payload size of Rank information, the number of usable resource elements in the PUSCH transmission, the modulation order, and the offset value of CQI/PMI and Rank, respectively. The purpose of this proposal is to reduce for example problems related to the fact that the code rate, at which the number of resource elements to be used by the control information, is derived from a “virtual” code rate of data, i.e. data and control are “rate matched” into the PUSCH, so the data encoded bits will actually never occupy the entire PUSCH.